As a material for springs used in the exhaust system of a car engine, austenitic stainless steels referred to as heat-resistant steels, such as SUS 304, SUS 316, and SUS 631J1 (JIS), or precipitation-hardened stainless steels have been used at an operating temperature of 350° C. or below.
In recent years, demands for more stringent control of the exhaust gas of automobiles have been increasing as a measure for environmental protection. The increasing demands have brought a tendency to raise the temperature of the exhaust system in order to increase the efficiency of engines and catalysts. Along with other parts, the springs are affected by this temperature rise. As a result, the most widely used austenitic stainless steels, such as SUS 304 and SUS 316, are sometimes insufficient in heat-resistant quality, particularly high-temperature tensile strength and high-temperature sag resistance especially needed for heat-resistant springs.
To avoid this problem, precipitation-hardened austenitic stainless steels such as SUS 631 are used as the material for the spring. However, the precipitation-hardened austenitic stainless steels have a problem in that a yield decrease in the hot working increases the cost, and an aging heat treatment at high temperatures for prolonged periods increases the production cost.
Consequently, the heat-resistant quality has been improved by employing the solid solution hardening which treats the steel by adding elements that form an interstitial solid solution, such as C and N, and ferrite-forming elements, such as W, Mo, V, Nb, and Si.
As a prior art that performs the solid solution hardening by adding the fore-going element, the published Japanese patent application Tokukoushou 54-18648 has disclosed a technique that intends to combine the anti-corrosion property of SUS 316 and the tensile strength of SUS 304.
Another published Japanese patent application, Tokukoushou 59-32540, has disclosed a technique in which in order to increase high-temperature tensile strength, high-temperature yield strength, and high-temperature oxidation resistance particularly at a temperature of 700° C. or so, the solid solution hardening is performed not only by the addition of C and N but also by the combined addition of B and V to an austenitic steel containing a large amount of Mn.
Yet another published Japanese patent application, Tokukaihei 4-297555, has disclosed a technique in which in order to attain high tensile strength and a long creep rapture life particularly at a temperature as high as 900° C. or so, the solid solution hardening is performed by the addition of C, N, Nb, W, etc.
Yet another published Japanese patent application, Tokukaihei 11-12695, has disclosed a technique which improves the performance of heat-resistant springs by mainly employing N to form a solid solution. Aiming at raising the elastic limit of SUS 316N, which is standardized in the Japanese Industrial Standard (JIS), by wire drawing, this technique has achieved not only a high elastic limit but also a high fatigue limit and a good heat-resistant quality at high temperatures by annealing a material containing a large amount of N.
Yet another published Japanese patent application, Tokukai 2000-239804, has disclosed a technique which achieves high sag resistance by the addition of elements, by the control of the average crystal-grain diameter in the γ phase (austenite) through the regulation of the heat-treatment conditions, and by the control of the aspect ratio (major-axis/minor-axis ratio) of the crystal grains in a longitudinal cross section of the wire through the regulation of the reduction rate of the cross-sectional area (hereinafter referred to as a “reduction of area”) at the time of wire drawing.
However, the three techniques disclosed by Tokukoushou 54-18648, Tokukoushou 59-32540, and Tokukaihei 4-297555 do not intend to improve the high-temperature sag resistance needed for heat-resistant springs at a temperature of 350 to 500° C., particularly at 400° C. or so. The technique disclosed by Tokukaihei 11-12695 limits the Ni equivalent in addition to the specification of the containing range of the material elements. However, the Cr equivalent, also, must be considered to stabilize the γ phase (austenite). This technique has a drawback of high production cost because it uses a large amount of costly Mo as an additive to a material based on SUS 316 containing a large amount of costly Ni. The method of controlling the structure disclosed by Tokukai 2000-239804 is insufficient in considering the conditions for the solution treatment and the reduction of area. As a result, uneven plastic deformation occurs locally, and the performance of the drawn material may not be improved.
The heat-resistant quality of the heat-resistant steel treated by the solid solution hardening with N varies with the heat-treating conditions and the reduction of area. In particular, when the solid solution hardening with N is performed, the degree of hardening depends largely on uneven plastic deformation caused by the coiling process, for example. Therefore, it is necessary to properly specify the structure and the production conditions in order to attain the high-temperature tensile strength and the high-temperature sag resistance needed for heat-resistant springs.